Java ConcurrentModificationException at list element access

Question

Can someone please explain to me why do I get a ConcurrentModificationException if I just want to access an element of the list?

I was under impression that ConcurrentModificationException is thrown only when one thread iterates over a collection and another one tries to modify it. This is not my case though. I just want to get an element of the list.

Here is a code snippet which reproduces my problem (please don't judge the logic, I just wanted to reproduce the problem):

private static void checkConcurrentModificationException(){

    List<String> stringSpan = new ArrayList<String>();
    List<List<String>> clausesStrings = new ArrayList<List<String>>();
    List<String> testStringList = new ArrayList<String>();

    for (int i = 0; i < 121; i++) {
        testStringList.add("abc");
    }

    for (int i = 0; i < testStringList.size(); i++) {
        stringSpan.clear();
        if (i%2==0) {
            if (i>3) {
                for (int j = i+1; j < testStringList.size(); j++) {
                    if (j>i+4 || j>=testStringList.size()-1) {
                        stringSpan = testStringList.subList(i, j); //might be the problem
                        clausesStrings.add(stringSpan);
                        break;
                    }
                }
            }
        } 
    }
    System.out.println("Clause list size: "+clausesStrings.size());
    System.out.println("First clause: "+clausesStrings.get(0));
}

I analyzed the instructions and came to a conclusion that the problem lies in the stringSpan = testStringList.subList(i, j); but I'm not quite sure what happens behind and why it fails. Also, I tried to copy(without reference) the resulting list of lists clausesStrings to a new one but no luck so far.

Answer 1

"I was under [the] impression that ConcurrentModificationException is thrown only when one thread iterates over a collection and another one tries to modify it." Nothing in the documentation says anything about CME happening only in a multithreaded scenario. How did you get that impression?

Give us the exact text, copy-pasted, of the exception message.

I suspect stringSpan.clear(); is your culprit. clear "Removes all of the elements from this list", and since stringSpan is a sublist, it's backed by the list over which you're iterating. So you also remove all those elements from testStringList, while you're iterating over it! Bam! ConcurrentModificationException.

EDIT:

For those who so eagerly downvoted my answer: TRY IT!

It works. When I ran the OP's code, CME! When I made the suggested changes,

run:
Clause list size: 58
First clause: [abc, abc, abc, abc, abc]
BUILD SUCCESSFUL (total time: 0 seconds)

NO CME! Take that, haters!

Here's the code:

public class SubModifier {
  static void checkConcurrentModificationException(){

    List<List<String>> clauses = new ArrayList<>();
    List<String> testItems = new ArrayList<>();

    for (int i = 0; i < 121; i++) {
      testItems.add("abc");
    }

    for (int i = 0; i < testItems.size(); i++) {
      if (i%2==0) {
        if (i>3) {
          for (int j = i+1; j < testItems.size(); j++) {
            if (j>i+4 || j>=testItems.size()-1) {
              List<String> span = testItems.subList(i, j);
              clauses.add(span);
              break;
            }
          }
        }
      } 
    }
    System.out.println("Clause list size: "+clauses.size());
    System.out.println("First clause: "+clauses.get(0));
  }

  public static void main(String[] args) {
    checkConcurrentModificationException();
  }
}